In the past, laser plasma filaments were generated due to the Kerr effect creating multiple axial foci along the propagation path of a high intensity laser beam. The high intensity beam breaks down the atmosphere and generates plasma at each focus point. This process is repeated over and over, causing the beam to focus, de-focus and focus again, generating a plasma filament. A disadvantage of this method is that the only parameter that is used to control the behavior (length, position, lifetime) of the plasma filament is the peak power, pulse length and geometrical focus of the laser.
Prior works have been done to attempt to overcome some deficiencies through the use of a method called “sub-aperturing” to create the location and intensity control where they divide the whole optical aperture radially (into rings of different radius) or rotationally (into triangle-like slices). Each focal location/intensity difference is imparted with these different slices of the optical aperture. The issue with this implementation is that this sub-aperturing causes the effective optical aperture of the propagating wavefront to be reduced, limiting the ability to propagate these types of beams over large distances and placing a lower limit on the spot size that can be achieved at focus. There are iterative holographic techniques that have been used to generate axial foci, but computer generated masks are (1) designed in a brute force manner and without the relationships between the physical effect and the necessary wavefront alterations, (2) there is no way to tune the system to account for any operating condition issues that may need adjustment to the mask. Other works have achieved similar results using intensity masks but these systems have a large amount of optical loss due to the absorptive manner in which the mask works. This is especially unsuitable for high power systems where absorption will cause the device to break. There is a need for an improved method of creating a plasma filament.